<style>
    div {
        height: 40px;
        width: 70%;
        background: black;
        margin: 50px auto;
        border-radius: 5px;
        position: relative;
    }

    #rect {
        animation-name: rainbow;
        animation-duration: 4s;
    }

    @keyframes rainbow {
        0% {
            background-color: blue;
            top: 0px;
            left: 0px;

        }
        50% {
            background-color: green;
            top: 50px;
            left: 25px;
        }
        100% {
            background-color: yellow;
            top: 0px;
            left: -25px;
        }
    }
</style>

<div id="rect"></div>